Interstellar comet 3I/ATLAS contains exceptionally high levels of methanol. This suggests it formed under conditions different from most comets in our solar system. The Atacama Large Millimeter/submillimeter Array (ALMA) observed the comet. These observations revealed that tiny icy grains around the comet release methanol. This provides a detailed look at an object from another star system.
Researchers used ALMA's Atacama Compact Array in Chile to observe 3I/ATLAS in late 2025. Sunlight heated the comet's icy surface as it moved closer to the Sun. Gas and dust escaped, creating a bright cloud around its core. Studying molecules in this cloud helped identify the comet's chemical makeup. This offers a rare opportunity to investigate how icy bodies form in other planetary systems.
The research team focused on methanol (CH3OH) and hydrogen cyanide (HCN). ALMA showed that 3I/ATLAS has an unusually high amount of methanol relative to hydrogen cyanide. Methanol-to-HCN ratios of approximately 70 and 120 were measured on two observation dates. These values place 3I/ATLAS among the most methanol-rich comets ever studied.
These measurements indicate that the ice within 3I/ATLAS formed or was exposed to conditions unlike those for most solar system comets. Earlier observations by the James Webb Space Telescope found that 3I/ATLAS had a coma dominated by carbon dioxide. The new ALMA measurements add abundant methanol to the comet's unusual chemical features.
ALMA's high-resolution imaging allowed tracking of how molecules are released. Hydrogen cyanide primarily originates from the comet's nucleus. Methanol, however, appears to come from both the nucleus and from ice particles within the coma. These small icy grains behave like miniature comets. As 3I/ATLAS nears the Sun, ice within the grains turns to gas, releasing more methanol.
Comet 3I/ATLAS is the third confirmed object from interstellar space. The other two are 1I/'Oumuamua and 2I/Borisov. Each interstellar object offers a chance to compare our solar system with other planetary systems. The unique chemistry of 3I/ATLAS adds to understanding how planets and comets form around distant stars.
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